1. Field of the Invention
The present invention relates to a method of operating an in-bath smelting reduction furnace.
2. Description of the Prior Art
In the in-bath smelting reduction method, a large amount of slag is caused to be present in the furnace as a medium for reducing molten ore. Ore supplied from the top or bottom of the furnace is melted into the metal bath or the slag in the furnace and is incorporated into the slag as iron oxides. The metal bath and slag are agitated to bring the iron oxides into contact with carbon present in the metal bath and carbon materials present in the slag in the form of coke or char, thereby reducing the iron oxides and producing hot metal.
The reduction of the ore requires a large amount of reduction heat. In the in-bath smelting reduction method, this heat is obtained by supplying oxygen or oxygen-containing gas into the furnace for burning fuel separately supplied thereto. As the fuel there are used materials containing carbon or hydrocarbons, such coal, coke and carbonized petroleum residue.
The main roles played by the slag in in-bath smelting reduction are that of shielding the hot metal bath from the oxygen, thereby preventing re-oxidization of the metal and promoting the reduction reaction in the slag, and that of circulating in the furnace so as to supply the combustion heat effectively to all parts of the furnace.
On the other hand, the carbon material suspended in the slag serves as a reducing agent for the molten iron oxides present in the slag and as a medium for conducting the combustion heat. Further, the carbon materials work to suppress the excessive foaming of the slag which is apt to occur because of the tendency of fine bubbles of gas generated in the slag to coalesce and, as a result, they help to prevent "slopping" (overflow of foamed slag at the furnace mouth), a phenomenon that makes continued operation impossible.
JP-A-62-224619, for example, discloses a method for efficiently conducting in-bath smelting reduction by supplying carbon material consisting of lump and powder materials mixed at a prescribed ratio to the slag so as to produce a high-temperature and strongly reductive atmosphere.
As will be understood from this, the amount of carbon material in the slag is highly important to stable in-bath smelting reduction operation. A number of methods have been proposed for measuring the slag carbon material content.
In one of these methods the amount of carbon material present in the slag is estimated from the residual C content of the furnace continuously calculated as the difference between the amount of C in the carbon material etc. supplied to the furnace and the total amount of C in the off-gas from the furnace. However, with this method a large discrepancy tends to arise between the actual and estimated amounts of carbon materials in the slag over long-term operation. This is due to the fact that there is ordinarily a 0.1-1% error in the measured value of supplied materials, a similar degree of error in the calculated off-gas flow rate and also some degree of error in the analysis of the components, and these errors accumulate over the passage of time.
In addition, the char formed when the volatile matter is driven out of the coal in the furnace is in large part made up of relatively small particles which tend to be entrained and carried off by the generated gas at the rate of at least 3% and, in some cases, up to 15%, and this also affects the amount of carbon material in the furnace. Since this rate of entrainment cannot be calculated instant by instant, such entrained carbon material also introduces a large error factor into the measurement of the carbon material present in the slag.
It is thus very difficult to maintain the carbon material content of the slag constantly at the ideal level so that it frequently becomes too high or too low, which gives rise to operational problems that will now be discussed.
When the carbon material content of the slag is insufficient, the slag swells excessively, giving rise to slopping so that the slag running over at the furnace mouth makes it impossible to continue the operation. On the other hand, when the carbon material content is excessive, the fluidity of the slag containing the carbon material is hindered and the excess carbon material reacts again with the combustion gas, reducing the gas and lowering the post combustion ratio. This means that the amount of heat generated per unit weight of the coal decreases and is found to cause a worsening of the unit consumption of coal and oxygen.
Thus when the carbon material of the slag cannot be maintained at an appropriate level, it either becomes impossible to continue the in-bath smelting reduction operation or becomes impossible to produce hot metal economically at a good unit consumption of coal and oxygen.
JP-A-61-221322 discloses a method in which post combustion heat is transferred to the slag in a converter-type vessel and the slag bath is agitated by gas for transferring the aforesaid heat to the molten metal. The agitating method used for promoting the heat transfer involves blowing gas into the slag and the molten metal.
JP-A-61-213310 discloses a method for increasing heat utilization efficiency when in-bath smelting reduction is carried out in a converter-type vessel that can be top blown. This is accomplished by establishing the conditions of: an amount of slag of not less that 250 kg/t, blowing bottom-blown gas at a rate accounting for 3-40% of the total amount of gas supplied, and maintaining the MgO +Al.sub.2 O.sub.3 content of the slag at not more than 23%.
These conventional techniques focus solely on operation for improving the rate of heat transfer and the reaction rate and are based on the simple concept that it suffices to achieve appropriate slag agitation. They betray inadequate attention to factors other than slag agitation, such as control of the agitation by bottom bubbling to within an appropriate range, suppression of the amount of dust generated, and the like.
Moreover, since research into the in-bath smelting reduction method has conventionally been conducted using very small experimental furnaces in the 1 to 10-ton range, the agitation gas flow rate per tuyere has been quite small, specifically in the vicinity of several tens to 100 N.sup.3 /h. As a result, the effect of increasing the gas flow rate per tuyere was completely unknown and no solutions were available for the problems that would arise when bottom-blown gas is introduced at a large flow rate, as is indispensable in the case of a large furnace.
As was pointed out earlier, the suspension of carbon materials in the slag is important in an operation using the in-bath smelting reduction method. Ordinarily, the amount of carbon material suspended is equivalent to 10-100 wt% based on the weight of the slag. However, this material tends to be entrained by the furnace gas and carried off, with up to 15-20% of the charged coal sometimes leaving the furnace in this manner. This loss of carbon material not only increases the unit consumption of the carbon material but also increases the risk of slopping because of the lower percentage of carbon material present in the slag.
A particular problem is the slopping that occurs when the carbon material dust loss becomes so large as to excessively reduce the percentage of carbon material present in the slag. In the worst cases, slopping will occur a mere 30 - 40 min after the start of operation and make further operation impossible. Because of this, there has been felt a particularly strong need for a method capable of reducing loss of carbon materials by furnace gas entrainment. Current in-bath smelting reduction furnaces employ refractory of the MgO-Cr.sub.2 O.sub.3, MgO-C or Al.sub.2 O.sub.3 type. At the lower part of the furnace (which is immersed in the metal bath or the slag bath), the operating temperature is relatively low (about 1500 C) but the agitating force of the bottom-blown gas is large, resulting in a refractory wear rate of about 1-4 mm/h. At the top of the furnace (where the gas burns), the high post combustion ratio raises the gas temperature to 1700.degree.-2000 .degree. C. or even higher. The refractory at this portion is further subjected to erosion by slag splashing. As a result, the refractory wear rate is high, today generally in the range of 3-10 mm/h.
For the in-bath smelting reduction method to be cost-competitive with the coke oven-blast furnace process, it is considered that the refractory wear rate must be reduced to 0.5-1 mm/h as an immediate target value.
The major cause for refractory wear is that caused by heat so that it should be possible to reduce the wear rate by lowering the operating temperature. JP-A-62-230908 discloses a low-temperature operating method in which dephosphorization is promoted during operation by maintaining the C content of the hot metal over 3.5% and maintaining the iron tapping temperature at least 200.degree. C. higher than the liquidus but not higher than 1450.degree. C.